Swimming in the cold waters off the coast of Alaska may be a far cry from the warm, muddy waters of the Ganges River in India and Bangladesh, but researchers have found a curious link between an ancient and modern species of dolphin. Buried in the collection of the Smithsonian Museum for over half a century, the skull of an ancient dolphin that lived in Alaska 25 million years ago might provide a glimpse into the evolution of a dolphin that still clings on in South Asia.

The newly identified fossil (which you can play with in 3D here) could give an insight into the evolution of the South Asian river dolphin (Platanista gangetica). “One of the most useful ways we can study Platanista is by studying its evolutionary history, by looking at fossils that are related to it to try to get a better sense of where it's coming from,” says Alexandra Boersma, one of the two researchers describing the fossil in PeerJ. From this, they could try and understand what caused the once diverse and widespread group of marine mammals to dwindle to just the single species that exists today.  

The fossil comes from a time when it's thought the two major groups of dolphins and whales seen today – toothed whales and baleen whales – first diverged. All living cetaceans fall into one of these groups, with the larger of the bunch, such as the blue whale, typically belonging to the baleen group, and the smaller dolphins being toothed (though, as always, there are a few outliers). By comparing the skull, which has been given the species name Akrtocara yakataga, with other dolphin species both alive and dead, they have determined that it's an ancient relative of the South Asian river dolphin.

The South Asian river dolphin, which is divided into two subspecies – one of which lives primarily in the Ganges and Brahmaputra Rivers and the other that is found mainly in the Indus River – are unusual beasts. Rare, secretive, and living in large muddy rivers, not much is known about them except that they tend to swim on their side, have no lens in their eyes (making them effectively blind), and rely almost entirely on echolocation.

One thing that is certain, however, is that they are in need of urgent help. As the last living decedents of a lineage of marine dolphins, the evolutionarily distinct mammals are heading towards extinction. But in a way, that makes the latest discovery even more perplexing. “Considering the only living dolphin in this group is restricted to freshwater systems in Southeast Asia, to find a relative that was all the way up in Alaska 25 million years ago was kind of mind-boggling,” says Boersma.

^{Image in text: Artistic reconstruction of a pod of Arktocara yakataga. Linocut print art by Alexandra Boersma}